1. Field of the Invention
Embodiments of the present invention relate generally to electrical circuit design and development, and, more particularly, to a method for flexible observability in a scan mode.
2. Description of Related Art
Computer circuitry has evolved from relatively simple, basic implementations to complex, high-speed designs. An increased demand for speed, features and capabilities of modern communications, computing and processing devices has driven computer circuitry to become faster and smaller. Faster and smaller circuit designs have been a challenge for designers who reach the limits of currently known design techniques and strategies. Developments in electrical circuit design have also increased the need for new methods of testing and functionality in scan chains.
During tests of electrical circuits, a scan chain may be used to determine the correctness of circuit functionality during a “scan.” Typically, a scan chain consists of a chain of one or more flip-flops through which values are scanned or shifted. The scan chain is typically fully scannable; that is, each flip-flop in the scan chain is controllable and may be observed during the test. In this manner, any fault or signal value on any flip-flop can be seen and used by a tester to control a cone of logic behind the flip-flop. However, such a configuration does not allow for observation-only/observe-only flip-flops in scan. Some schemes use a shadow flip-flop to hold the input of a non-scannable flip-flop, where the non-scannable flip-flop is not in the scan chain, an inherent drawback. These test and scan variations lack a scannable flip-flop implementation (i.e., a scan chain of scannable flip-flops) where the flip-flop may be in an observe-only mode.